This invention relates to induction loop vehicle detectors, and in particular to such detectors in which the induction loop conductors are supported in a conduit having pressure-withstanding couplings.
The need to detect, count, monitor or control vehicles on highways, bridges, parking lots, garages, street intersections, and shopping centers is well established. The primary use of such devices is the control of vehicles at intersections with traffic lights. That is, turn lanes and cross traffic can be controlled on an as needed basis with the use of built-in traffic monitoring devices. In most common use today are inductive loop detectors wherein a generally horizontal loop, or a plurality of loops, of a conductor are placed in the traffic lane and connected to control equipment. By applying current to the conductor, inductance is generated which is altered by the presence of a vehicle.
Conventionally, most loops are installed by cutting or sawing a one-eighth inch to three-eighths of an inch wide by a one inch to three inch deep saw cut in the pavement The resulting channel typically is filled with epoxy, rubber, plastic, wax, or asphalt based compounds after the conductor is laid in it. Such methods have not been found to be entirely acceptable because the failure rate is fairly high. Failure results when the channel filler separates from the channel walls allowing moisture to enter, or the abrading and ultimate breaking or shorting of conductors at the channel corners of the saw cuts. This can also be due to deterioration in the pavement adjacent the saw cut, or to vibration caused by road equipment and heavy traffic near or adjacent the slot.
Durability of the loop can be improved by placing the conductors in a groove in a base layer of asphalt or concrete which is then overlayed with a top layer to seal the conductors. However, ultimately, it is found that similar types of failures result due to pavement movement and deterioration and vibration of the conductors in the asphalt for the reasons previously mentioned.